It appears Niukore was either re-signed without an announcement, or his initial contract had an extra year in there (perhaps an option?) which was likewise never announced. Leleisiuao, Mahoney and Stone were also already contracted for next year.

Anyway, there's a new list of off contract merkins (i.e. they are uncontracted for 2020). That list looks like this (age next year in parentheses):